A goal of a vehicle stability control system is to stabilize yawing, rolling, and lateral drifting motions of a motor vehicle during all driving conditions, on all road surfaces, and within a full range of driving speeds. A system capable of achieving such a goal is called an integrated stability control (ISC) system.
Existing stability control systems were initially developed to attenuate undesired vehicle yawing motion and are known as electronic stability control (ESC) systems. Improvements to ESC systems went on to include roll and lateral drifting directional stabilization and are known as roll stability control (RSC) systems. The roll stability control (RSC) system achieves roll and lateral drifting stabilizations for a vehicle driven on road surfaces that have high friction levels whereby on-road rollover may be possible.
Stability control systems, such as RSC systems, are typically equipped with a motion sensor cluster, or set, that may include a roll rate sensor, a yaw rate sensor, a longitudinal accelerometer and a lateral accelerometer. RSC systems use a roll rate sensor and control algorithms in addition to the standard ESC systems to enhance vehicle state estimation, thereby refining the control performances. Refined control performance is mainly due to the fact that the added roll sensing leads to a more accurate total vehicle roll angle sensing such that lateral drifting, also known as side slipping, may be detected from the on-board lateral accelerometer. Side slipping may be detected as soon as the actual motion induced lateral acceleration exceeds a predetermined level of uncertainty associated with the lateral accelerometer sensor, such as, for example, when a signal-to-uncertainty ratio (SUR) for the lateral acceleration is large. The lateral acceleration sensing uncertainties may result from the sensor noise, the sensor zero-crossing drift, the sensor scaling factor nonlinearity, the sensor crossing axis sensitivity, etc., as well as any unmeasured road bank uncertainties. Similar sensing uncertainties may be present for the other sensor elements.
However, an unstable condition may also happen for a low sensing signal-to-uncertainty ratio driving event. For instance, driving on a snowy and/or icy surface, the vehicle's cornering acceleration level is usually low yet the vehicle may still enter into a large drifting/side slipping motion. In this case, the actual cornering acceleration is very close to the accelerometer sensing uncertainty level, i.e., the sensing signal-to-uncertainty ratio is close to 1. Lateral instability may be determined from a sideslip angle computed from the longitudinal and lateral vehicle body velocities, which are estimated from the first integrals of accelerations together with the double integrals of angular rates. When the sensing signal-to-uncertainty ratio is close to 1, the integral of the sensing uncertainty could dominate the estimation. Hence it is important to remove sensing uncertainties and boost the sensing signal-to-uncertainty ratio. One way to do this is to enhance the sensing capability so as to differentiate road influence from the sensing uncertainty. For instance, a vertical accelerometer and/or a pitch rate sensor may be added to the sensor set used in roll stability control in order to remove sensing uncertainties due to the road influence.
While it is highly desirable to enhance sensing capability to remove the sensing uncertainties for vehicle states with low signal-to-uncertainty driving conditions, there is a need to develop a more cost effective approach than adding costly sensors.